1. Field of the Invention
The present invention relates to a technique of a light emitting device using a light emitting element, and more specifically to a technique of a light emitting device controlling an applied voltage of the light emitting element with electric field effect type transistors.
2. Description of the Related Art
In recent years, the development of a display device for displaying an image has been progressed. As the display device, a liquid crystal display device for displaying an image using a liquid crystal element has been widely used for a display screen of a mobile telephone by taking advantages of a high image quality, a thin type, a light weight, and the like.
On the other hand, in recent years, the development of a light emitting device using a light emitting element has been also progressed. The light emitting device has features such as a high response speed, superior moving picture display, and wide viewing characteristic in addition to advantages of the existing liquid crystal display device. Thus, it has been noted as a next-generation compact mobile flat panel display capable of using moving picture contents.
The light emitting element is made of broad materials such as an organic material, an inorganic material, a thin film material, a bulk material, or a dispersion material. Of them, as a typical light emitting element, there is an organic light emitting diode (OLED) mainly made of an organic material. The light emitting element has a structure in which an anode, a cathode, and a light emitting layer sandwiched between the anode and the cathode are provided. The light emitting layer is made of one or plural materials selected from the above-mentioned materials. Note that the amount of current flowing between both electrodes of the light emitting element is in direct proportion to light emission luminance.
In many cases, a plurality of pixels each having a light emitting element and at least two transistors are provided in the light emitting device. In each of the pixels, a transistor connected in series with the light emitting element (hereinafter indicated as a driver transistor) has a function for controlling light emission of the light emitting element. When a gate-source voltage (hereinafter indicated as VGS) of a driver transistor and a source-drain voltage (hereinafter indicated as VDS) thereof are changed as appropriate, the driver transistor can be operated in a saturation region or a nonsaturation region.
When the driver transistor is operated in the saturation region (|VGS−Vth|<|VDS|), the amount of current flowing between both electrodes of the light emitting element is greatly dependent on a change in |VGS| of the driver transistor but not dependent on a change in |VDS|. Note that a drive method of operating the driver transistor in the saturation region is called constant current drive. FIG. 9A is a schematic view of a pixel to which the constant current drive is applied. In the constant current drive, a gate electrode of the driver transistor is controlled to flow the necessary amount of current into the light emitting element. In other words, the driver transistor is used as a voltage control current source and the driver transistor is set such that a constant current flows between a power source line and the light emitting element.
On the other hand, when the driver transistor is operated in the nonsaturation region (|VGS−Vth|>|VDS|), the amount of current flowing between both electrodes of the light emitting element is changed according to both values of |VGS| and |VDS|, more specifically, |VDS| is changed depending on the value of |VGS| and in a range within 1V at the maximum. Note that a drive method of operating the driver transistor in non-saturation region is called constant voltage drive. FIG. 9B is a schematic view of a pixel to which the constant voltage drive is applied. In the constant voltage drive, the driver transistor is used as a switch, and a power source line and the light emitting element are shorted if necessary, thereby flowing a current into the light emitting element.
There is provided a light emitting device capable of displaying clear multi-gradation colors using pixels which perform such constant voltage driving. Further, there also is provided a light emitting device applicable to a time-gradation method. (see Patent References 1 and 2).    [Patent Reference 1] JP 2001-343933 A    [Patent Reference 2] JP 2001-5426A
Light emitting elements by nature increase their resistance (internal resistance) with time. An increase in internal resistance causes reduction in amount of current flowing between anodes and cathodes of light emitting elements because the current is in reverse proportion to the resistance. In short, the luminance of a light emitting element is lowered with time and this makes it difficult to obtain a desired light emission luminance.